This invention relates to a method of co-producing dialkyl carbonate and alkanediol and, in particular, to a method of co-producing dialkyl carbonate and alkanediol through the use of a catalyst which is a complex salt compound.
Various homogeneous catalysts have been proposed for carbonate transesterification. For example, U.S. Pat. Nos. 3,642,858 and 4,181,676 disclose the preparation of dialkyl carbonates by transesterifying alkylene carbonates with alcohols in the presence of alkali metals or alkali metal compounds without the use of a support material. U.S. Pat. No. 4,661,609 teaches the use of a catalyst selected from the group consisting of zirconium, titanium and tin oxides, salts or complexes thereof.
Commercial use of homogeneous catalysts is restricted because separation of the catalyst from the unconverted reactants and organic product can be difficult. Because the transesterification is an equilibrium reaction, in an attempt to isolate the intended dialkyl carbonate by distillation of the reaction liquid without advance separation of the catalyst, the equilibrium is broken during the distillation and a reverse reaction is induced. Thus, the dialkyl carbonate once formed reverts to alkylene carbonate. Furthermore, due to the presence of the homogenous catalyst, side reactions such as decomposition, polymerization, or the like concurrently take place during the distillation which decrease the efficiency.
Various heterogenous catalysts have also been proposed for carbonate transesterification. The use of alkaline earth metal halides is disclosed in U.S. Pat. No. 5,498,743. Knifton, et al., "Ethylene Glycol-Dimethyl Carbonate Cogeneration," J Molec. Catal. 67:389-399 (1991) disclose the use of free organic phosphines or organic phosphines supported on partially cross-linked polystyrene. U.S. Pat. No. 4,691,041 discloses the use of organic ion exchange resins, alkali and alkaline earth silicates impregnated into silica, and certain ammonium exchanged zeolites. U.S. Pat. No. 5,430,170 discloses the use of a catalyst containing a rare earth metal oxide as the catalytically active component. The use of MgO is disclosed in Japanese Unexamined Patent Application 6[1994]-107,601. The use of MgO/Al.sub.2 O.sub.3 hydrotalcites is disclosed in Japanese Unexamined Patent Application 3[1991]-44,354. Zeolites ion-exchanged with alkali metal and/or alkaline earth metal, thereby containing a stoichiometric amount of metal, are disclosed in U.S. Pat. No. 5,436,362.
European Patent Application 0 478 073 A2 discloses a process for producing a dialkyl carbonate by contacting an alkylene carbonate with an alkanol in the presence of a mixed metal oxide, i.e., a catalyst containing two or more metal oxides. Unlike the process disclosed in the European application, the method of the invention does not utilize a mixed metal oxide catalyst. Rather, the method of the invention utilizes a catalyst which is a single complex salt compound.
Inorganic heterogenous catalysts generally possess thermal stability and easy regeneration. However, these catalysts, including the zeolites containing a stoichiometric amount of alkali or alkaline earth metal, generally demonstrate low activity and/or selectivity and are unsatisfactory for commercial application. Polymer supported organic phosphines and ion exchange resins show high activity and good to excellent selectivity in transesterification reaction between alkylene carbonate and alkanol; however, these polymeric materials do not appear very stable and gradually lose catalytic activity, especially at relatively high temperatures.
Thus, there remains a need for a method of transesterifying alkylene carbonate with alkanol to co-produce dialkyl carbonate and alkanediol which will provide higher feed conversion and product selectivity over a wide temperature range.